In the field of thin film fabrication, the substrate generally needs to be heated to control the growth of the thin film, thereby adjust the physical and chemical properties of the film. The conventional heating methods are as the followings: (1) Heating through thermal conduction. Namely, the substrate is pasted to a thermal source and the heat is directly transferred from the thermal source to the substrate. This heating method requires that the substrate is closely contacted to the thermal source to assure the uniform and consistent conduction of the heat. Consequently, there are two shortcomings of this heating method. One is that the size of the substrate can't be too large in order to avoid the inhomogeneous heating resulting from the poor contact between substrate and thermal source. The other is that it's complicated to achieve the specific motion of the large-size substrate in order to fabricate large-scale uniform thin film; (2) Heating through thermal radiation. Namely, the substrate is placed in the radiation range of a high-temperature source (the resistive wires, halogen lamps, etc.) and heated through absorbing the rays (main the infrared ray) generated by the source. It requires that the amount and distribution of the high-temperature source should be carefully considered to achieve the high enough and uniform temperature in the substrate. Compared to method (1), the advantage of this method is that the substrate can be free and be of larger size, while the disadvantage is that the high-temperature source is more difficult to design and needs more space; (3) Heating by induction. Namely, the substrate is placed in the high-frequency electromagnetic field, which can induce a current with same frequency in the substrate, and then heated by the Joule effect. Compared to method (1) and (2), it's a shortcoming that the substrate used in this method must be conductive and be of regular shape and uniform resistivity. In addition, this manner raises much higher requirements on the high-frequency source and the electromagnetic shielding of the setup to prevent the electromagnetic field from harming the surrounding human bodies and electronic devices. However, it's superior to heat the substrate fast and can achieve the substrate surface temperature of 800˜1000° C. in a few seconds.
Although the above heating methods are of their own merits and drawbacks, it's same that the energy used for heating substrate is transferred from other high-temperature objects or transformed from other forms of energy. During the transferring or transforming process, only a small part of the energy is practically utilized to heat substrate and the rest part of the energy is wasted by the thermal objects or the electric source themselves. To prepare the thin film on long and thin metal tape, the above heating method (1) is improper because such metal tape is flexible, especially for the moving metal tape. Although the heating method (2) can be applied to heat this kind of metal tape substrate, the design of the thermal source, such as its amount and distribution, should be carefully considered to achieve the uniform distribution of the temperature along the metal tape length and width. Generally, the design is complicated. If the heating method (3) is applied, the frequency should be high enough to restrict the skin effect, thereby improve the energy efficiency. In addition, the plasma would be easily stimulated in the vacuum chamber under the coupling effect of high-frequency electrical field at the state of high frequency. Consequently, the temperature can't be accurately controlled under the simultaneous action of the induction heating and the plasma. Compared with the above three heating methods, it is much simpler and more energy-efficiency to heat the metal tape substrate by the Joule heat generated from itself. In detail, a current supplied by a power source is conducted into the metal tape substrate and the Joule heat will be generated because of the resistant of the metal tape itself, thereby the metal tape is heated. To date, several similar heating methods have been reported in the patents (CN ZL01110150.4 and US 20140033976A1) to heat such metal tape. In patent CN ZL01110150.4, the current flows into and out of the metal tape through electrodes which are quite tightly contacted to the surface of the tape ends, thus this manner can only be used to heat static metal tape. In patent US 20140033976A1, the current is conducted into another metal tape rather than the tape needing to be coated. Therefore, the metal tape substrate is still heated through absorbing the radiation of other thermal metal tape, which is same with heating method (2) and is certainly of the drawbacks of heating method (2). Furthermore, the heating manner in US 20140033976A1 is also inconvenient to simultaneously deposit thin films on both faces of the metal tape substrate.
Hence, this invention proposes the following novel heating method and system to heat flexible metal tape substrate, concerning the drawbacks of the existing heating methods.